Short-sightedness was rare. In Asia, it is becoming ubiquitous

In the early 1980s Taiwan’s army realised it had a problem. More and more of its conscripts seemed to be short-sighted, meaning they needed glasses to focus on distant objects. “They were worried that if the worst happened [ie, an attack by China] their troops would be fighting at a disadvantage,” says Ian Morgan, who studies myopia at Australian National University, in Canberra. An island-wide study in 1983 confirmed that around 70% of Taiwanese school leavers needed glasses or contact lenses to see properly.

Don’t be short-sighted about this

For most of those affected, myopia is a lifelong, expensive nuisance. But severe myopia can lead to untreatable vision loss, says Annegret Dahlmann-Noor, a consultant ophthalmologist at Moorfields Eye Hospital, in London. A paper published in 2019 concluded that each one-dioptre worsening in myopia was associated with a 67% increase in prevalence of myopic maculopathy, an untreatable condition that causes blindness. (A dioptre is a measure of a lens’s focusing power.) In some parts of East Asia, 20% of young people have severe myopia, defined as -6 dioptres or worse (see chart 2). “This is storing up a big problem for the coming decades,” says Kathryn Rose, head of orthoptics at the University of Technology, Sydney.

Since there is no obvious way in which learning sums, spelling or even the Talmud could cause short-sightedness directly, the assumption was that education was a proxy for something else. One possibility is the popular notion (raised by Johannes Kepler, a German astronomer who himself needed glasses, more than 400 years ago) that myopia is linked to too much close-in work, such as reading and writing.

That theory remains popular, says Dr Rose, but evidence for it is mixed at best. Instead, the dominant hypothesis now is that exposure to daylight is the main variable. A study of Californian children, published in 2007, found that time spent outdoors was strongly associated with a lower risk of myopia. Another paper, published the following year by Drs Rose and Morgan and their colleagues, followed more than 4,000 children in Sydney for three years and came to a similar conclusion. The type of activity—sports, walking, picnics—did not seem to matter. Simply being outdoors was the crucial point. The researchers cross-checked the close-work hypothesis and found that being outside drastically reduced the risk of short-sightedness, even for children that did a lot of it.

This theory fits the data neatly. It explains why myopia seems, like diabetes and heart disease, to be what doctors call a “disease of affluence”—more common in rich countries than poor ones—since economic growth brings with it more education, and therefore, for children, more time inside. It explains why rates are high in East Asia in particular, says Dr Morgan, since the ubiquity of private tutoring and after-school classes mean schoolchildren there routinely work longer days than their Western counterparts. Most South Korean students, for instance, attend private tutoring schools called hagwons in which lessons often stretch well into the evening.

Seeing the light

The daylight-exposure theory is also bolstered by animal studies, in which that exposure can be carefully controlled, and in which dimness reliably produces short sight. Researchers have a putative mechanism, too. Exposure to bright light appears to stimulate the production of dopamine, a neurotransmitter, in the retina. Dopamine, in turn, seems to help regulate the rate at which the eye grows. Too little, and the eye grows too long to focus properly.

Human trials, too, confirm the theory. One of the biggest, led by Pei-Chang Wu of the Chang Gung University College of Medicine, in Taiwan, was published in 2020. It reported results from millions of Taiwanese primary-school pupils who had gone through the school system between 2001 and 2015. In 2010 the government instituted a programme called “Tian-Tian Outdoor 120”, which encouraged schools to take pupils outside for two hours a day. After it was implemented, rates of myopia fell, slowly but steadily, from 49.4% in 2012 to 46.1% in 2015—reversing a decades-long trend of rising rates.

Exactly how much light is needed is unclear, though Dr Morgan reckons 10,000 lux is in the right ballpark. That is about as much as you might get in the shade on a reasonably sunny day. (Direct sunlight in the tropics can exceed 100,000 lux.) Levels indoors, by contrast, rarely exceed 1,000 lux. It is technically possible to light classrooms to 10,000 lux, notes Dr Rose. But even with led lighting, she says, the amount of heat produced would require specialist air conditioning, and the glare might be enough to make reading tricky.

Researchers have also been working on ways to slow myopia’s progression once it has started. One is to use low doses of atropine, a poisonous chemical found in deadly nightshade—the juice of which was once used by women to dilate their pupils, in order to make themselves look more attractive. Another is specially designed “ortho-k” contact lenses, which are intended to reshape the cornea while worn. (The cornea is the front, transparent part of the eye, which does most of the work of focusing light on the retina; the eye’s so-called lens is mainly for fine tuning.) These also seem effective, though Dr Rose worries about the side-effects from giving contact lenses to children, since if misused they can cause irreversible corneal scarring.

Sophisticated pairs of spectacles may help, too. In 2020 the bmj published a Chinese trial of “defocus incorporated multiple segments” (dims) glasses. The lenses of these have a central zone designed to correct the wearer’s sight, which is surrounded by hundreds of other small zones of different optical power. The idea is to provide both clear vision through the central part of the lens and deliberately distorted vision through the smaller zones, since poorly focused vision is thought to signal to the eye to slow its rate of growth. Wearing dims glasses appeared to cut the rate of myopic progression roughly in half.

Eyedrops, more sunlight and clever glasses might be able to prevent or slow myopia in future generations of schoolchildren. But by the time sufferers reach adulthood, the condition is permanent. This means that in some countries, a public-health problem is already baked in. For those with really serious myopia, says Dr Rose, and who are at the highest risk of bad complications, unhealthy changes in the eye can start to occur when patients are in their 40s. “And some of those are not in any way treatable.” ■

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